Hi
Mine is also a 2004 and I thought that it had the alarm as warning light came on and off with ignition as it should.
Upon closer inspection the coolant bottle had no sensor float,it came out the factory without one and the wiring neatly cable tied out the way.(MG Rover )
I had to buy a new sensor expansion bottle and all works well.
Please just check yours as it would be a bummer to rely on something that will never work as intended.